1. Field of the Invention
The present invention relates to a continuous process for the preparation of lower dialkyl carbonates by catalysed reesterification of ethylene glycol carbonate or propylene glycol carbonate with lower alcohols, the reaction of the alkylene carbonate with a dialkyl carbonate-containing alcohol being performed in counter-current. The process requires the introduction of pure alcohol below the point of introduction for the dialkyl carbonate-containing alcohol.
2. Description of the Related Art
The preparation of dialkyl carbonates from ethylene glycol carbonate and alcohols, in which the underlying ethylene glycol is formed simultaneously, is known and has been described repeatedly. However, almost all descriptions are concerned with the catalysis of this reesterification, while the industrial realisation of this process is not described in detail. However, in U.S. Pat. No. 4,691,041 as one of the few sources, there is a detailed description of such a process:
Ethylene glycol carbonate and methanol in a molar ratio of 1:4 are pumped at about 100.degree. C. and about 7 bar through a tube filled with a basic ion exchanger, in the most expedient case the reesterification equilibrium being established, which contains, in addition to methanol as the main component and unreacted ethylene glycol carbonate, the reaction products dimethyl carbonate and ethylene glycol and additionally by-products, such as polyglycols and dimethyl ether.
The reaction mixture described is then separated in a first distillation apparatus into the high-boiling fractions ethylene glycol carbonate, ethylene glycol and polyglycols on the one hand and the low boilers methanol, dimethyl carbonate and dimethyl ether on the other hand. In a second distillation, the high boilers are separated into an ethylene glycol containing up to 10% of ethylene glycol carbonate and a polyglycol-containing ethylene glycol carbonate. The polyglycol-containing ethylene glycol carbonate flows back into the reesterification reaction, where the risk of concentration of the high-boiling by-products must be expected. The ethylene glycol carbonate-containing glycol is fed into a hydrolysis in which the glycol carbonate which has reached the ethylene glycol is cleaved into glycol and CO.sub.2 and is thus lost as a source of carbon dioxide.
From the low-boiler fraction mentioned, in a 3rd distillation which is carried out at elevated pressure, a bottom phase of pure dimethyl carbonate and a dimethyl carbonate-depleted methanol, which additionally contains dimethyl ether, is then obtained. The dimethyl carbonate-containing methanol is likewise returned to the reesterification reaction. By the recycling of carbonate-containing material, the space-time yield of re-esterification product is markedly reduced. This reduction is estimated to be 30 to 40%, compared with the initial filling of the reesterification apparatus. This reduction is all the more painful, since in any case the best which can be expected is the establishment of the reesterification equilibrium. These reductions are extremely restrictive for an industrial implementation. Therefore, although the return of a dialkyl carbonate-containing alcohol appeared to be highly desirable, since in such a case an extensive, and thus expensive in terms of energy, separation of the dialkyl carbonate and the fundamental alcohol would not have to be carried out, such a return did not appear to be realisable. The avoidance of such an expensive separation of carbonate and alcohol is desirable to the same extent for all lower alcohols (C.sub.1 -C.sub.4); however, it is particularly desirable in the dimethyl carbonate/methanol system which forms an azeotrope which is difficult to separate.
The use of a dialkyl carbonate-containing alcohol in an industrial realisation is nevertheless surprisingly expediently possible if the reesterification is carried out in a column apparatus in counter-current, a dialkyl carbonate-containing alcohol stream being led from the bottom towards the alkylene carbonate delivered in the upper part of the column, pure alcohol being additionally introduced below the introduction of the dialkyl carbonate-containing alcohol.